Japanese Patent Unexamined Published Application No. 2006-120686 (Kokai) (patent document 1) discloses an LSI mounting board, the object of which is to improve the yield ratio of acceptable LSIs (Large-Scale Integrated circuits), by saving LSIs that might be abolished as defective products because they do not satisfy a frequency specification requirement or a power specification requirement due to a variation in quality or variety factors in each LSIs, and in which a circuit is provided in the LSI for detecting a fluctuation in performance due to a variation in quality in each LSIs and the output value of this circuit is employed to control a voltage supplied to the LSI. The LSI mounting board is designed such that it can satisfy both of the specification requirements for the operating frequency and the power consumption, by setting a voltage applied to an LSI, which is regarded as a defective product because its power consumption is too large although there is still a surplus in the operating frequency, so that the voltage is lower than a suitable standard value, or by setting a voltage applied to an LSI, which is regarded as a defective product because the operating frequency is low although there is still a surplus in the power consumption, so that the voltage is higher than a suitable standard value.
Specifically, an LSI mounted on the LSI mounting board includes a processor, a ring oscillator, an LSI operation velocity measurement circuit, a power control circuit and a memory control circuit. In addition to the LSI, a clock oscillator, a voltage-variable power supply circuit, a ROM and a voltage-fixed power supply circuit are provided on the LSI mounting board. The clock oscillator generates an operating clock signal for the LSI. A clock signal generated by the clock oscillator is transmitted to the individual circuits of the LSI. The voltage-variable power supply circuit supplies power to the LSI. ROM stores therein a calculated flag and the velocity value. The LSI operation velocity measurement circuit measures the output frequency of the ring oscillator. Thus, the operating velocity fluctuation value of the LSI due to a manufacturing variation or variety factors in LSIs can be obtained. Using these circuits, it is possible to know how many times in a predetermined period the output of the ring oscillator rises.
The power control circuit is composed of a register, a counter and a comparator. A velocity value required for setting the optimal voltage to be applied is set on the register. The velocity value set on the register is generated, using a value that corresponds to the operating velocity of the LSI. This velocity value can be read from or written to the processor. The counter increments a value each time a clock signal rises. The comparator compares the output of the register with the output of the counter, and outputs a power voltage control signal. The power voltage control signal is transmitted to the voltage-variable power supply circuit. When the counter value is smaller than the velocity value outputted by the register, the power voltage control signal goes to level H (logical high), or when the counter value is equal to or greater than the velocity value, the power voltage control signal goes to level L (logical low). The voltage outputted by the voltage-variable power supply circuit can be adjusted by changing the duty ratio of the power voltage control signal.
The processor reads the velocity value from the ROM, and writes the velocity value to the register. Through this process, an appropriate voltage to be applied to an LSI can be set for each port.
An LSI having the maximum operating frequency higher than that in the specification and power consumption larger than that in the specification may possibly turn into a good product, by reducing a voltage applied to the LSI so that the power consumption for the LSI becomes lower than that in the specification requirement. Furthermore, an LSI having the maximum operating frequency lower than that in the specification requirement and power consumption smaller than that in the specification requirement may possibly turn into a good product, by increasing a voltage to be applied needed so that the maximum operating frequency becomes higher than that in the specification requirement.
However, since the process for calculating the applied voltage is performed only once at the time of product shipping inspection, and the value obtained at this time is thereafter employed, only the initial change of the performance due to a variation or variety factors through the manufacturing process can be coped with, while a time-transient change that occurs following the product shipping can not be handled. Further, since a processor, a ROM and the like are required, the circuit would become complicated and large.